The present invention relates to an ultrasonic bonding method and an ultrasonic bonding apparatus for mounting a chip device, such as a surface acoustic wave device (SAW) or a semiconductor chip, on a resin substrate or the like by ultrasonic bonding.
The weight, thickness and the size of electronic apparatus have rapidly been reduced. It leads to a fact that a face-down mounting techniques is an important technique with which chip devices, such as surface acoustic wave devices (SAW) or semiconductor chips, can be mounted on a substrate at a high density.
The face-down mounting method is exemplified by an ultrasonic bonding method with which bump electrodes of the chip device are joined to pad electrodes formed on a ceramic substrate by applying ultrasonic waves and a load. The foregoing method is an advantageous method because solder or adhesive agent is not required, thermal stress which is exerted on the chip device can be reduced even if the size of the chip device is reduced and satisfactory bonding strength can be increased. A fact is known that raising of the temperature to 150xc2x0 C. to 225xc2x0 C. when the ultrasonic bonding operation is performed raises the bonding strength (refer to, for example, Japanese Patent Laid-Open No. 10-107078).
The ceramic substrate is unsatisfactory in the processability and cost efficiency. Therefore, resin substrates, such as BT resin substrates or heat-resisting glass epoxy substrates, have been employed in recent years to mount the chip devices. When the resin substrate is employed, face-down of the connecting electrode of the chip device to, for example, a bump pattern provided for the resin substrate as the circuit electrode must be performed. Any detailed investigation about the conditions under which the ultrasonic bonding operation suitable for the resin substrate is performed has not been carried out. Therefore, the conventional ultrasonic bonding conditions employed for the conventional ceramic substrate have been employed. That is, adjustment of the temperature which is raised when the ultrasonic bonding operation is performed has not been performed. Hitherto, adjustment of the output of ultrasonic waves and pressure which must be applied has been performed. Therefore, the ultrasonic bonding operation has been performed at a temperature considered to be suitable without any consideration of the characteristic of the resin.
Recently a multiplicity of bump electrodes serving as connecting electrodes are provided for the joining surface of the chip device. When the bump electrodes are provided by disposing, for example, a multiplicity of balls, the heights of the bump electrodes cannot easily be uniformed. It is difficult to simultaneously join a multiplicity of bump electrode having various heights on the substrate having a rough surface. To overcome the dispersion in the nonuniform heights, an anisotropic conductive and adhesive layer has been employed. That is, adhesive layer is applied to the leading ends of the bump electrodes or a solder paste which is fused with heat is used or conductive powder is mixed with the resin to forcibly introduce the bump to establish the connection.
As a result, the attempt to increase the bonding strength results in excessive enlargement of the output of ultrasonic waves and the applied load during the joining operation. Thus, there arises problems in that the pad portion of the chip device is cracked, the pad of the chip device is separated from the chip device and the multiplicity of the bump electrodes provided for the chip device cannot uniformly be joined owing to an adverse influence of the rough surface of the resin substrate.
Moreover, the following problems must be solved.
(1) When the pattern on the circuit substrate on which the chip devices are mounted is fined (for example, a fine pattern of 10 xcexcm) or when the size of the chip device is reduced (for example, 1 mmxc3x971 mm), stability of the bonding operation must be improved in a case where a micro bump electrode (for example, diameter of 50 xcexcm) or a micro pad (for example, 100 xcexcmxc3x97100 xcexcm) is employed.
(2) Residual stress in the bump electrode, the pad, the circuit substrate or the base of the chip device caused from the ultrasonic bonding operation must be minimized.
(3) When a pyro-electric material, such as the SAW or the light modulating device, or dielectric material, such as birefringent optical material, is employed, mechanical shock (distortion and impact) and thermal shock (difference in the temperature and rate at which the temperature is changed) caused from the ultrasonic bonding operation must be reduced. Thus, stress which is exerted on the joint portion must be reduced to prevent, for example, discharge breakdown.
In view of the foregoing, an object of the present invention is to provide an ultrasonic bonding method with which optimum ultrasonic bonding conditions for a resin substrate can be determined when a chip device is mounted by a face-down method using a resin substrate as the substrate on which the chip device is mounted. The object of the present invention includes realization of ultrasonic bonding which permits a wide margin in the bonding operation, which is capable of realizing stable joining and which exhibits satisfactory reliability for a long time.
Another object of the present invention is to provide an ultrasonic bonding apparatus which is capable of face-down bonding a chip device on a resin substrate, which exhibits a wide margin in the bonding operation, with which stable joining can be realized and which exhibits satisfactory reliability for a long time.
According to first aspect of the invention, there is provided an ultrasonic bonding method arranged to mount a chip device on a resin substrate by performing ultrasonic bonding such that the position of a connecting electrode of the chip device is aligned to the position of a circuit electrode provided for a resin substrate having elastic modulus xcex5r at room temperature and elastic modulus xcex5h realized when the resin substrate is heated, the ultrasonic bonding method comprising: a step of heating the resin substrate to a temperature at which the ratio of the elastic modulus xcex5r at room temperature and the elastic modulus xcex5h realized after heating satisfies 1 greater than xcex5h/xcex5rxe2x89xa70.5 when the circuit electrode and the connecting electrode are ultrasonic-bonded to each other.
The elastic modulus of the resin substrate having satisfactory rigidity at room temperature is reduced owing to applied heat, causing the resin substrate to be flexible. Moreover, the reduction in the elastic modulus is limited to the half value of the elastic modulus realized at the room temperature. Therefore, when the bonding operation is performed such that the position alignment between the circuit electrode and the connecting electrode is performed and ultrasonic waves or a load is exerted, excessive deformation of the resin substrate can be prevented. Thus, energy of the ultrasonic waves, the load and heat are exerted in between the circuit electrode and the connecting electrode. Moreover, the resin substrate is adequately deformed to moderate the mechanical shock caused from the bonding operation. In particular, when a multiplicity of circuit electrodes and connecting electrodes are simultaneously ultrasonic-bonded, the flexibility of the resin substrate causes the joined portion to be deformed. Thus, uniform bonding of a multiplicity of the points can be performed even if the joined portion has a rough surface.
Preferably, temperature raised when the resin substrate is ultrasonic-bonded satisfies a range from 80xc2x0 C. to 170xc2x0 C.
Preferably, an amount of amplitude of ultrasonic waves arranged to be imparted on the chip device when the ultrasonic bonding operation is performed satisfies a range from 0.8 xcexcm to 1.6 xcexcm.
Preferably, the chip device is a surface acoustic wave device having a dielectric substrate, and the connecting electrode is a bump electrode formed on the dielectric substrate and made of gold or a gold alloy.
Preferably, the resin substrate is a thermosetting resin substrate, and the circuit electrode is constituted by forming a plated layer on the surface of a copper foil pattern, and the uppermost layer of the plated layer is made of gold.
Further, according to second aspect of the invention, there is provided an ultrasonic bonding apparatus arranged to ultrasonic-bond a connecting electrode of a chip device to a circuit electrode provided for a resin substrate to mount the chip device on the resin substrate, the ultrasonic bonding apparatus comprising: resin-substrate heating means for heating the resin substrate to a temperature at which the ratio of elastic modulus xcex5r of the resin substrate at room temperature and elastic modulus xcex5h realized after heating satisfies 1 greater than xcex5h/xcex5rxe2x89xa70.5.